Bioinspired cobalt cubanes with tunable redox potentials for photocatalytic water oxidation and CO2 reduction

Bioinspired cobalt cubanes with tunable redox potentials for photocatalytic water oxidation and CO2 reduction Zhishan Luo, Yidong Hou, Jinshui Zhang, Sibo Wang and Xinchen Wang* Full Research Paper Address: State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, China Email: Xinchen Wang* - * Corresponding author Keywords: CO2 reduction; cobalt cubane; photocatalysis; water oxidation; water splitting Open Access Beilstein J. Org. Chem. 2018, 14, 2331–2339. doi:10.3762/bjoc.14.208 Received: 14 May 2018 Accepted: 17 August 2018 Published: 05 September 2018 This article is part of the thematic issue "Photoredox catalysis for novel organic reactions". Guest Editor: P. H. Seeberger © 2018 Luo et al.; licensee Beilstein-Institut. License and terms: see end of document. Abstract The development of efficient, robust and earth-abundant catalysts for photocatalytic conversions has been the Achilles’ heel of solar energy utilization. Here, we report on a chemical approach based on ligand designed architectures to fabricate unique structural molecular catalysts coupled with appropriate light harvesters (e.g., carbon nitride and Ru(bpy)32+) for photoredox reactions. The “Co4O4” cubane complex Co4O4(CO2Me)4(RNC5H4)4 (R = CN, Br, H, Me, OMe), serves as a molecular catalyst for the efficient and stable photocatalytic water oxidation and CO2 reduction. A comprehensive structure–function analysis emerged herein, highlights the regulation of electronic characteristics for a molecular catalyst by selective ligand modification. This work demonstrates a modulation method for fabricating effective, stable and earth-abundant molecular catalysts, which might facilitate further innovation in the function-led design and synthesis of cubane clusters for photoredox reactions. Introduction The direct conversion of solar energy into chemical fuels (e.g., H2, CO and hydrocarbons) through water splitting and carbon fixation reactions is a sustainable solution to environmental concerns and long-term access to adequate energy supplies [1-7]. To realize these reactions, extensive studies have focused on the design and synthesis of chemically stable lightharvesting antenna materials and efficient cocatalysts, and their assembly in integrated artificial photosynthetic systems [8-13]. However, such target reactions are typical thermodynamically uphill reactions with large overpotentials, leading to low conversion efficiency. Therefore, the search for suitable cocatalysts to reduce the multielectron involved kinetic barriers for water oxidation and CO2 reduction is regarded as a critical step toward artificial photosynthesis, which can boost the photoconversion efficiency (PCE) significantly [14-19]. 2331 Beilstein J. Org. Chem. 2018, 14, 2331–2339. Molecular catalysts with complex and varied structural motifs are a class of promising catalysts for solar energy conversion, because of their well-controlled functions and tunable nature [20,21]. Their topologies and electron structures can be precisely engineered by ligand design, using the full arsenal of organic chemistry [22,23]. These unique structures benefit not only tailoring their redox and kinetic properties for catalysis, but also providing valuable structural information to understand the mechanistic insights of catalytic behavior [24-27]. In addition, the molecular catalysts can either be dissolved in liquids affording a homogeneous catalytic system [28,29], or immobilized on solid surfaces for application in heterogeneous catalysis [30-33], owing to their molecular nature with flexible ligand architectures [34,35]. In this regard, extensive attention has been contributed to the design and synthesis of molecular catalysts [36]. Unfortunately, most of the high-activity molecular catalysts are typically based on noble metals (e.g., Ru, Ir) [37-40], which seriously restricts their practical applications. Therefore, the development of effective, stable and sustainable molecular catalysts based on earth-abundant elements is highly desirable [41-43]. Inspired by the molecular Mn4CaO5 cubane of oxygen-evolving complex in photosystem II, there is an emerging number of molecular cubanes with metallic and heterobimetallic cores that are designed and synthesized for photosynthesis and electrochemistry. Cobalt-based molecular catalysts [44], in particular the ones containing a cubical Co4O4 core were studied extensively as energy conversion catalysts, because of their cubical topology that is structurally analogous to the biological Mn 4 CaO 5 cubane [45,46]. Driess et al. have reported the smallest possible molecular building block “Co4O4” cluster with a singly deprotonated dipyridyldiol (LH) as a chelating ligand [47]. Generally, Co4O4-based molecular catalysts can be easily tuned by ligand design, owing to their molecular nature [48,49]. For example, Hill et al. demonstrated that using polytungstate ligands to stabilize “Co4O4” cubane units can produce a robust homogeneous catalyst for solar water oxidation [50]. After that, Berlinguette et al. reported that replacing the inorganic ligand with an organic ligand, such as the pentadentate Py5 ligand can also well stabilize the “Co4O4” unit to catalyze water oxidation [51]. This finding is very important, which means there is ample choice of organic ligand architectures to tailor the electronic properties of the “Co4O4” unit for catalysis. In this regard, Nocera et al. selected an organic ligand bearing an electron-withdrawing group (fluorine) to optimize the “Co4O4” cubane unit for electrocatalytic water oxidation [52]. As expected, the resultant catalyst exhibited a larger catalytic current and an earlier onset potential with respect to its analogs without a fluorine functional group. Thus, the control of catalytic properties via molecular design by tunable ligand substitution is essential in the development of Co4O4-based cubane catalysts. However, most of the researches focused on the oxidative properties of the Co4O4 core [53], and its use for reduction reactions is rarely covered. Theoretically, the redox potential of Co 4 O 4 cubane clusters should be tuned by virtue of different ligand substitutions, thus it is highly possible to develop a Co4O4-based catalyst for reduction applications, such as H2 evolution and CO2 fixation. Herein, we demonstrate that molecular Co 4 O 4 cubanes (Figure 1) are readily and precisely manipulated to tune their redox functions through regulating their electronic structures by ligand engineering. The use of electron-withdrawing or donating ligands can easily adjust their catalytic properties for water oxidation and CO 2 reduction, respectively. For example, organic ligands with strong electron-withdrawing groups (R = CN, Br) enhance their oxidation capability for water oxi- Figure 1: (a) Molecular structures of the Co4O4 cubane catalysts. (b) Ball-and-stick representation of complex 1-H; H atoms (...truncated)